1. Field of the Invention
The present invention relates to coating compositions comprising a liquid vehicle and a carbon product having a t-area greater than or equal to 400 m2/g. The present invention further relates to coating compositions comprising a liquid vehicle and a modified carbon product having a t-area greater than or equal to 350 m2/g.
2. Description of the Related Art
Coating compositions are used for decorative, protective, and functional treatments of many kinds of surfaces. These surfaces include coils, metals, appliances, furniture, hardboard, lumber and plywood, marine, automobile, cans, and paperboard. Some coatings, such as those on undersea pipelines, are for protective purposes. Others, such as exterior automobile coatings, fulfill both decorative and protective functions. Still others provide friction control on boat decks or car seats. Some coatings control the fouling of ship bottoms, others protect food and beverages in cans. Silicon chips, printed circuit panels, coatings on waveguide fibers for signal transmission, and magnetic coatings on video tapes and computer disks are among many so-called hi-tech applications for coatings.
Surface coating compositions are generally more or less viscous liquids with three base components: a film-forming substance or combination of substances called the binder, a pigment or combination of pigments, and a volatile liquid. The combination of binder and volatile liquid is called the vehicle. Vehicles may be in a solution form or as a dispersion of fine binder particles in a non-solvent. Pigments are finely divided, insoluble, solid particles dispersed in the coating vehicle and are distributed throughout the binder in the final film. Surfactants may also be added and are typically used as pigment dispersants. The components and manufacturing of coating compositions such as aqueous coatings are further discussed in the Concised Encyclopedia of Polymers, Science and Engineering, pages. 160-171 (1990), which is incorporated herein by reference.
Pigments in coating compositions provide opacity and color. The amount and type of pigment controls such properties as the gloss of the final film and can have important effects on its mechanical properties. Some pigments even inhibit corrosion. Further, pigments affect the viscosity and enhance the application properties of the coating. Carbon products and, in particular, carbon black, are common pigments used in coating applications.
An important variable determining the performance of carbon products in coating compositions is surface area. It is well known in the art that the higher the surface area of a carbon product in a coating composition, the better the color properties of the resulting coating (see, for example, the Cabot Corporation Technical Report S-140 entitled xe2x80x9cBlack Pearls(copyright) 1400, Monarch(copyright) 1400: Superior High Color Carbon Blacksxe2x80x9d). Surface area, which is inversely related to the size of the particles, is known to effect such properties as gloss, jetness, and bluetone.
There are several different measures of the surface area. One common technique is to measure the amount of a probe material that is capable of being absorbed onto the carbon surface. Typical probes molecules are nitrogen (known as the BET method), iodine, and cetyltrimethylammonium bromide (CTAB).
Different probe molecules result in different surface area values and can reflect different aspects of the carbon surface. For example, CTAB and iodine surface areas are dependent on the chemistry of the carbon surface. Two carbon blacks with the same particle size can have very different CTAB and iodine values if their surface chemistries are different. Also, BET surface area is dependent on the porosity of the pigment. Carbon surfaces generally contain pores. The total surface area of a pigment (which is measured by the BET method) is therefore the sum of its internal surface area (from pores) and its external surface. Thus, two pigments may also have the same particle size yet may have very different BET surface areas due to their porosity. The t-area (also known as the statistical thickness surface area, or STSA) is a measure of only the external surface area of a carbon product and is calculated by subtracting the porosity value from the BET value. As a result, the t-area of a carbon product is always less than the BET value.
As stated above, a goal for the coating supplier is to provide a coating with the best overall color properties. In general, smaller particle pigments are desired in order to obtain these results. However, smaller particle size (higher surface area) pigments also results in an increase in the viscosity of the coating composition. Also, and perhaps more importantly, the particle size and surface area of the pigment effects its dispersibility into the coating composition. In manufacturing coatings, it is desirable to disperse the pigment in such a way as to achieve a stable dispersion where most, if not all, of the pigment particles are separated into the individual particles. The mechanism of dispersion of a pigment involves wetting, separation, and stabilization. It is known that the higher the surface area of a pigment, the more difficult that pigment is to wet and therefore disperse in the vehicle used for the coating composition. A poor pigment dispersion leads to a deterioration of coating properties. Dispersion stability may also suffer. High surface area pigments often require high energy processes (such as milling) to obtain stable dispersions and therefore good color performance. For these reasons, commercially available pigments for high color coating applications are designed to afford the best compromise of surface area and dispersion quality and stability.
One method to prepare coating compositions with improved properties is described in U.S. Pat. Nos. 5,672,198 and 5,713,988, both incorporated in their entireties herein by reference, which disclose aqueous and non-aqueous inks and coatings containing modified carbon products having attached organic groups. While the foregoing compositions have yielded good coatings, a need remains for improved compositions with high color performance and good overall application and mechanical properties.